1. Field of the Invention
The present invention relates generally to rotating electromagnetic machines such as generators and synchronous motors, and more particularly, to a system and method for monitoring the insulation resistance of a machine winding.
2. Description of Related Art
In large rotating electromagnetic machines, a rotor winding, which is isolated from ground, is supplied with a current from an exciter. The rotor itself is grounded, and the rotor winding is encased by insulation such that it is electrically isolated from the rotor. Ground faults, however, sometimes occur. For example, the resistance of the winding insulation to ground may decrease to an extent that produces a low resistance path between the rotor winding and ground. Such insulation faults can result in damage to the rotor. Accordingly, large rotating electromagnetic machines are typically equipped with ground detection systems that monitor the rotor winding insulation. Often, an alarm is activated when an insulation fault is detected so the machine may be shut down and repairs made.
Several shortcomings exist with known rotor winding insulation monitoring systems, however. For example, prior art systems typically only provide a binary indication of the ground fault the alarm is simply activated upon detection of a fault. There is no resistance measurement to give an operator an indication of an impending fault, allowing a repair before a damaging fault occurs. Many prior art systems are prone to false alarms, which can result in significant costs when the machine, such as a large generator, is shut down unnecessarily.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the invention, a method of determining the resistance of a winding insulation includes applying a pulsed voltage having a maximum value and a minimum value to the winding insulation. The values of the maximum and minimum current flowing through the winding insulation are determined, and the winding insulation resistance is calculated based on the difference between the maximum and minimum current. A ground fault may be detected based on the calculated winding insulation resistance. In exemplary implementations of the method, determining the maximum and minimum current flowing through the winding insulation includes calculating the current flowing through a sense resistor coupled in series with the winding insulation. Additionally, in exemplary embodiments, the pulsed voltage maximum value and minimum value both are negative values with respect to the winding negative terminal.
In other aspects of the invention, a winding insulation monitoring system includes a sense resistor connected to the winding insulation and a pulsed voltage source is coupled to the sense resistor to apply a pulsed voltage having maximum and minimum values to the winding insulation via the sense resistor. A processing device has input terminals coupled to the sense resistor and is programmed to determine the resistance of the winding insulation. This may include calculating the difference between the minimum and maximum current flowing through the sense resistor. In certain exemplary embodiments, the maximum and minimum values of the pulsed voltage are both negative values.
In accordance with still further aspects of the invention, a generator system employing a winding insulation monitor is disclosed The generator system includes a rotor with a field winding situated therein. Insulation isolates the field winding from the rotor. An exciter circuit is connected to the field winding. The winding insulation monitor includes a stationary portion having a receive antenna and an inductive loop coupled to a power source. A rotating portion is mounted to rotate with the rotorand includes a sense resistor connected to the winding insulation, a processing device having input terminals coupled to the sense resistor, a transmit antenna connected to an output terminal of the processing device, and a pick up coil for inductive coupling with the inductive loop to power the rotating portion.
A pulsed voltage having maximum and minimum values is applied to the winding insulation via the sense resistor, and the processing device determines the resistance of the winding insulation based on the pulsed voltage. As noted above, the maximum and minimum values of the pulsed voltage are both negative values in exemplary embodiments of the invention. To determine the resistance of the field winding, the processor may be programmed to calculate the current flowing through the sense resistor.